While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep UV lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has recently attracted a great deal of attention utilizes as the deep UV light source a high-intensity KrF excimer laser and an ArF excimer laser featuring a shorter wavelength. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004). There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemically amplified resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemically amplified resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
In chemically amplified, positive working, resist compositions to be developed with alkaline developers, a resin and/or compound in which alkali-soluble phenol or carboxylic acid is partially or entirely protected with acid-labile protective groups (acid labile groups) is catalytically decomposed by an acid which is generated upon exposure, to thereby generate the phenol or carboxylic acid in the exposed area which is removed by an alkaline developer. Also, in similar negative working resist compositions, a resin and/or compound having alkali-soluble phenol or carboxylic acid and a compound (acid crosslinking agent) capable of bonding or crosslinking the resin or compound under the action of an acid are crosslinked with an acid which is generated upon exposure whereby the exposed area is converted to be insoluble in an alkaline developer and the unexposed area is removed by the alkaline developer.
On use of the chemically amplified positive resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator or PAG) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask having a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemically amplified positive resist compositions adapted for KrF excimer laser generally use phenolic resins, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Typical PAGs used therein are iodonium salts, sulfonium salts, bissulfonyldiazomethane compounds, N-sulfonyloxydicarboxylmide compounds, and O-arenesulfonyloxime compounds. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
PAGs capable of generating 10-camphorsulfonic acid and 2,4,6-triisopropylbenzenesulfonic acid, whether they are sulfonium and iodonium salts or O-arenesulfonyloxime compounds, are low diffusible and very useful in providing high resolution resist materials. See JP-A 5-222257, JP-A 9-323970, JP-A 10-39500, and JP-A 2004-133393.
As the requisite pattern size is reduced, however, even the use of these PAGs encounters problems including low resolution and low stability to the environment. Improvements in resolution may be made by incorporating an acid labile group which is more scissile to acid in the base resin, by using a basic additive, or by controlling processing conditions.
The environmental stability is generally divided into two categories. One stability problem is that the acid generated upon exposure is deactivated with airborne bases on the resist film or bases on the substrate beneath the resist film. This phenomenon is often found when a photoacid generator capable of generating an acid having a high acid strength is used. This problem is addressed by rendering the acid labile groups in the resin more scissile to acid or by reducing (or weakening) the acid strength of the acid generated. The other problem of environmental stability is that when the duration between exposure and post-exposure bake (PEB) is prolonged, that is, in the case of post-exposure delay (PED), the acid generated diffuses through the resist film so that acid deactivation occurs if acid labile groups are less scissile or acid decomposition reaction occurs if acid labile groups are more scissile, often leading to variations of the pattern profile. In chemically amplified positive resist compositions having acid labile groups including primarily acetal groups, for example, the line width of unexposed areas is often narrowed.
JP 3613491 discloses an anion-bound PAG polymer having PAG combined with a non-acid-labile group-containing monomer. Since the effect of PAG is weakened by the non-acid-labile group-containing monomer, the composition is unsatisfactory in resolution or the like.
As discussed above, in order to gain a higher resolution, an acid labile group which is more scissile must be incorporated in the base resin and the PAG is desired to generate a low diffusible acid. JP-A 2006-58842 and JP-A 2007-304528 describe as the agent for deactivating fluorinated alkanesulfonic acid generated by a PAG upon exposure, a sulfonium salt capable of generating 1-adamantanesulfonic acid having lower acid strength than the fluorinated alkanesulfonic acid. In these patents, the sulfonium salt is used as adjuvant rather than the PAG.
The electron beam (EB) lithography is not only of interest as the micropatterning technology capable of processing to a feature size of 0.1 μm or less, but also becomes indispensable to form mask patterns. However, EB imagewise drawing takes a longer time than the conventional block exposure process. To increase the throughput, resists are thus required to have a higher sensitivity. The stability with time of resist in vacuum during and after imagewise drawing is also one of crucial performance factors. Some coatings on substrates, for example, coatings of SiO2, TiN or Si3N4 on silicon wafers and chromium oxide on mask blanks can affect the resist profile after development (e.g., forming a footing profile) depending on the particular type of substrate. For achieving high resolution and maintaining a profile after etching, it is one of important performance factors that the pattern profile of resist is kept rectangular independent of the substrate type.